Publications (140)209.29 Total impact
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Article: Statistical features of coherent structures at increasing magnetic field pitch investigated using fast imaging in QUEST
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ABSTRACT: Fluctuations in slab plasma produced by electron cyclotron waves in an open magnetic configuration with vertical (Bz) and toroidal (Bt) fields are investigated as a function of Bz/Bt (field pitch) using a high speed visible camera in the spherical tokamak QUEST. Higher order moments (skewness s and kurtosis k) of spatio-temporal fluctuations are investigated. Background fluctuations, generation of coherent convective structures (blobs) and their propagation are analyzed at the intensity gradient (Rim) and the source free (essentially vacuum on the low field side) regions respectively. Parabolic relation ( ) is observed between s and k on the entire R-Z plane for a wide range of Bz/Bt. Progressive shift from the Gaussian statistics beyond Rim is observed with the increasing Bz/Bt. From the moments of the PDFs, possible location for blob generation is inferred to be Rim. Direct correspondence is observed with the prominence of the blobs and increase in stochastic forces with the increase in field pitch. Distribution of waiting time suggests that the blob generation may be a Poisson process. Accelerated cross field transport is observed for reasonably large sized blobs.Nuclear Fusion 10/2012; · 4.09 Impact Factor -
Conference Proceeding: Non-inductive Current Start-up and Plasma Equilibrium with an Inboard Poloidal Field Null by Means of Electron Cyclotron Waves in QUEST
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ABSTRACT: Non-inductive current start-up via relativistic electron cyclotron resonance interaction is investigated for the high ratio (~10 %) of vertical Bv to toroidal Bt fields and the concave field lines in the QUEST spherical tokamak. In the start-up scenario with an internal poloidal field null (IPN), the fast current start-up rate of 0.3-0.5 MA/sec and correlation with mildly relativistic electrons accelerated due to multiple ECR interaction are observed. In steady state high p equilibrium characterized by the inboard null (Rs ~ 0.7×R0) and p of 1.5 is achieved, where p are the inverse aspect ratio and poloidal beta, respectively. Relaxation oscillations in this equilibrium and confinement of the energetic electrons are discussed.24th IAEA Fusion Energy Conference, San Diego, USA; 10/2012 -
Article: Fast visible imaging and edge turbulence analysis in QUEST.
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ABSTRACT: A fast visible imaging system is installed on the spherical tokamak QUEST to study edge turbulence. The camera uses a complementary metal-oxide semiconductor detector with a maximum resolution of 1024 × 1024 at 7000 frames∕s (fps) and can achieve 775 kfps at a resolution of 128 × 24. In this paper, we present the salient features of the system and its application to study edge turbulence in 8.2 GHz ECRH driven slab plasma, without plasma current. Vertical magnetic field (B(z)) topology is varied with three sets of poloidal field (PF) coils and the variation in the edge turbulence is investigated as a function of the B(z) strength and curvature. Fluctuation amplitude was highest for the shallow PF well. Cross-correlation coefficient shows distinct coherent mode along z direction at the steep density gradient region and it grows with the PF mirror ratio.The Review of scientific instruments 10/2012; 83(10):10E524. · 1.52 Impact Factor -
Article: Fast visible imaging and edge turbulence analysis in QUEST
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ABSTRACT: A fast visible imaging system is installed on the spherical tokamak QUEST to study edge turbulence. The camera uses a complementary metal-oxide semiconductor detector with a maximum resolution of 1024×1024 at 7000 frames/s (fps) and can achieve 775 kfps at a resolution of 128×24. In this paper, we present the salient features of the system and its application to study edge turbulence in 8.2 GHz ECRH driven slab plasma, without plasma current. Vertical magnetic field (Bz) topology is varied with three sets of poloidal field (PF) coils and the variation in the edge turbulence is investigated as a function of the Bz strength and curvature. Fluctuation amplitude was highest for the shallow PF well. Cross-correlation coefficient shows distinct coherent mode along z direction at the steep density gradient region and it grows with the PF mirror ratio.Review of Scientific Instruments 01/2012; · 1.37 Impact Factor -
Article: Analysis of PWI footprint traces and material damage on the first walls of thespherical tokamak QUEST
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ABSTRACT: After several non-inductive current startup experimental campaigns in the spherical tokamak QUEST, its metallic first walls have revealed various kinds of damages as a signature of strong plasma wall interaction (PWI). Several types of footprint traces, namely colored regions formed due to material erosion/redeposition, melting of plasma facing components (PFCs) and numerous arc tracks on the chamber walls are recognized. Analysis of the re-deposited materials on collector probes is carried out using Xray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). Redeposition of several impurity materials such as carbon, oxygen and tungsten is identified. The footprint traces are majorly formed on the lower side PFCs, showing a large up/down asymmetry. Both toroidally symmetric and asymmetric footprint traces are formed on the bottoms side divertor plate and the lower part of the outboard side walls, respectively. Localized melting occurred on the outboard side limiters is attributed to the loss of energetic electrons produced via electron cyclotron resonance (ECR) heating. The observed damages are discussed in view of localized PWI, loss of energetic electrons, particle drifts, sputtering, arcing and redeposition of eroded materials. Material analysis and numerically calculated guiding center orbits of the charge particles are used to discuss these damages.Fusion Engineering and Design 01/2012; 87:77. · 1.49 Impact Factor -
Article: HYDROGEN PERMEATION MEASUREMENTS IN THE SPHERICAL TOKAMAK QUEST AND ITS NUMERICAL MODELING
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ABSTRACT: A permeation measuring system with a nickel membrane of 30 µm thickness was installed near the mid plane of the spherical tokamak, QUEST. Hydrogen permeation through the membrane heated at fix temperatures (422 – 506 K) was measured during short pulse (< 1 s) and long pulse (1 hour) plasma discharges. After the membrane was heated to a required temperature, hydrogen plasma was discharged using a 2.45 GHz or 8.2 GHz RF system. Significant plasma-driven permeation was observed even for very short plasma discharges (e.g. 0.1s). Numerical calculations with the use of diffusion equation under recombination boundary conditions were conducted to simulate the transient permeation behavior. The numerical calculations were also used to estimate diffusion coefficient and recombination coefficients of membrane material. Temperature dependence of both the coefficients was explained by the Arrhenius law. A one hour long permeation curve was also numerically reproduced using the same set of parameters except an increasing recombination coefficient on the plasma side of the membrane.Fusion Science and Technology 11/2011; 60:1511. · 1.12 Impact Factor -
Article: Non-Inductive Start up of QUEST Plasma by RF Power
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ABSTRACT: Both start-up and sustainment of plasma were successfully achieved by fully non-inductive current drive using microwave with a frequency of 8.2 GHz. Plasmas current of 15 kA was implemented for 1 s. Magnetic surface reconstruction exhibited a plasma shape with an aspect ratio of below 1.5. The plasma current was dependent significantly on the launched microwave power and vertical magnetic field, while not affected by the mode of launched wave and the toroidal refractive index. Hard X-ray (HXR) emitted from energetic electrons accelerated by the microwave was observed, and the discharge with a plasma current over 4 kA followed the same trend as the number of photons of 10 keV to 12 keV. This suggests that the plasma current may be driven by energetic electrons. Based on the experimental conditions, alternative explanations of how the plasma current could be driven are discussed.Plasma Science and Technology 06/2011; 13(3):307. · 0.41 Impact Factor -
Article: Numerical studies of internal disruptions in Heliotron E
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ABSTRACT: Internal disruptions found by soft-X-ray and electron cyclotron emission measurements can be explained in terms of a non-linear evolution of the unstable m = 1/n = 1 internal kink mode and subsequent reconnection of the magnetic field lines due to finite resistivity. The linear growth rate is found to be proportional to for a fixed external rotational transform, where rs denotes the position of the inner q = 1 surface.Nuclear Fusion 01/2011; 23(12):1669. · 4.09 Impact Factor -
Article: Heating and confinement studies of electron cyclotron resonance heated plasmas in Heliotron E
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ABSTRACT: Radiofrequency (RF) heating of currentless plasmas is performed at the fundamental electron cyclotron frequency. The absorbed power profile is determined from the decay of the electron internal energy profile immediately after the RF pulse is turned off. Measurements of the absorbed power profile at various positions of the cyclotron resonance layer are compared with single-pass absorption profiles predicted by a ray tracing code. The profiles of electron temperature, density and ion temperature are studied as a function of the absorbed power, its radial profile and the electron line averaged density. Finally, a scaling of the energy confinement time obtained by regression analysis is compared with a scaling for neutral beam heated plasmas.Nuclear Fusion 01/2011; 28(10):1801. · 4.09 Impact Factor -
Article: Ion energy containment in Heliotron-E
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ABSTRACT: First measurements of the charge-exchange ion temperature in the Heliotron-E device are described. The limiting temperatures calculated from a simple energy balance equation are presented and compared with the measured temperatures. In low-current plasma, the measured temperatures agree well with the thermal conduction limit based on neoclassical theory, which includes an effect of the electron-to-ion temperature ratio Te/Ti. In high-current plasma, however, the ion temperature is about one-third of the limiting temperature. This discrepancy may be due to anomalous heat and particle transport driven by the plasma current.Nuclear Fusion 01/2011; 22(10):1341. · 4.09 Impact Factor -
Article: Neutral-beam injection heating experiment on currentless plasma in the heliotron E device
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ABSTRACT: Recent results of the neutral-beam heating experiments (NBI) on the Heliotron E plasma are reported. The target plasma for NBI is produced by electron cyclotron resonance heating (ECRH), using two gyrotrons (53.2 GHz, 200 kW × 2). The neutral beams of 2.9 MWmax are injected 20 to 40 ms afterwards in order to produce a high-energy currentless plasma. At a magnetic field of 1.9 T, a maximum ion temperature of 1.0 keV is obtained at a density of 1.9 × 1013 cm−3. The maximum eτE is 2.4 × 1012 cm −3s. By reducing the magnetic field to 0.94 T and using the second-harmonic resonance of microwaves to produce a target plasma, a high-beta plasma is produced by NBI. The maximum central-beta and volume-average beta values are 3.6% and 2%, respectively. The paper describes the results of NBI heating experiments and the confinement and MHD activity properties of the plasmas in question.Nuclear Fusion 01/2011; 24(12):1551. · 4.09 Impact Factor -
Article: Studies of currentless, high-beta plasma in the Heliotron E device
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ABSTRACT: A currentless plasma with a volume-averaged beta value of 2% has been produced with neutral beam heating. Target plasmas were created by second harmonic resonance heating with electron cyclotron waves (150–350 kW and 53.2 GHz) at a magnetic field strength of 0.94 T. Neutral beam injection (23–30 keV and 1.3−2.6 MW) was used to heat the plasma further. MHD stable and unstable high-beta plasmas were observed. The Q-mode plasmas were produced with the help of intense neutral gas puffing. Properties of the MHD activity and confinement of high-beta plasmas are discussed and compared with theoretical studies.Nuclear Fusion 01/2011; 25(12):1783. · 4.09 Impact Factor -
Article: Scalings of energy confinement and density limit in stellarator/heliotron devices
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ABSTRACT: The paper presents a study of empirical scaling of energy confinement observed experimentally in stellarator/heliotron devices (Heliotron E, Wendelstein VII-A, L2, Heliotron DR) for plasmas heated by electron cyclotron heating and/or neutral beam injection. The proposed scaling of the gross energy confinement time is: , where P is the absorbed power (MW), n is the line average electron density (1020 m−3), B is the magnetic field strength on the plasma axis (T), a is the average minor radius (m) and R is the major radius (m). The empirical scaling of the density limit obtainable under the optimum condition is proposed to be: . These scalings for helical systems are compared with those in tokamaks. The energy confinement scaling has a similar power dependence as the L-mode scaling of tokamaks. The density limit scaling for helical systems seems to indicate an upper limit of the achievable density similar to that in many tokamaks. From the energy confinement time and the density limit , a beta limit can be derived: , which can be lower than the stability/equilibrium beta limit. Thus, from the viewpoint of designing a machine, the values of B, a and R should be selected with care because the dependence of the confinement time (or nτET) and of the above beta limit on these values is different.Nuclear Fusion 01/2011; 30(1):11. · 4.09 Impact Factor -
Article: Density fluctuations in currentless high beta plasmas in Heliotron E
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ABSTRACT: The stability properties of currentless high beta plasmas have been investigated over a wide range of density profiles ne(r), by varying the gas puffing conditions and by pellet injection. Line density fluctuations correlated with a pressure driven MHD instability were measured by a multi-channel far-infrared laser interferometer. When an increase in the plasma beta was accompanied by a peaking of ne(r), line density fluctuations with a frequency of 2–5 kHz periodically appeared. The repetition time of the fluctuations was typically 2–5 ms. The poloidal and toroidal mode numbers of the fluctuations were identified as m=l and n=l. For plasmas with an extremely peaked profile, density fluctuations corresponding to the m=3/n=2 mode were observed. With the growth of this mode, internal disruptions occurred. Because of these disruptions, the profile of ne(r) rapidly changed from a peaked one to a broader one. The normalized average radius, r/a, of the phase reversal in the density profile variation caused by the disruption is about 0.5, which roughly corresponds to the ι =2/3 surface, ι being the rationalized rotational transform. It is most likely that the large amplitude mode with m=3/n=2 triggers the internal disruptions. The onset regions for MHD instability have also been investigated for various density profiles and beta values.Nuclear Fusion 01/2011; 27(6):895. · 4.09 Impact Factor -
Article: Transport analysis of injected impurities in currentless Heliotron E plasmas
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ABSTRACT: Impurity transport in currentless Heliotron E plasmas is examined. The main aim of the experiment is to clarify the density dependence of impurity transport both in electron cyclotron heated (ECH) and neutral beam injected (NBI) plasmas. In experiments with a stationary electron density, the confinement time of impurities is determined as a function of the line averaged electron density. The inherent difference between ECH and NBI plasmas, observed in an earlier measurement, seems to have been masked by the operating conditions. From a direct comparison between NBI plasmas and simultaneous ECH and NBI heated plasmas of the same density, a pumping-out effect of impurities by the ECH pulse is established. From numerical analyses, the unexpected discrepancy between observed and calculated VUV spectral data can be explained by a charge exchange process. The observed impurity transport is found to be well described by a model which incorporates the effects of radial diffusion and inward convection. The inward flow does not seem to be due to the plasma potential because the flow is also observed in ECH plasmas where the heating power is expected to contribute to a reversed potential. The flow velocity is of the order of the classical friction between background ions and seems to play only a minor role in various aspects of impurity transport in NBI plasmas, such as impurity accumulation. The variety in impurity transport in Heliotron E plasmas is best understood by changes in the diffusion coefficient.Nuclear Fusion 01/2011; 27(7):1075. · 4.09 Impact Factor -
Article: Transport of injected impurities in Heliotron E
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ABSTRACT: Impurities have been injected into Heliotron E in order to study the transport properties of currentless plasmas. In ECH-produced plasmas, the observed impurity transport can be described by purely anomalous diffusion, with a coefficient of the order of D = 2000–5000 cm2 s−1. During neutral beam operation, the impurity transport can be characterized by considerably weaker diffusion, combined with inward convection. Convection velocities as large as a few hundred centimetres per second are inferred.Nuclear Fusion 01/2011; 24(9):1205. · 4.09 Impact Factor -
Article: Pellet injection experiment on NBI current-free plasmas in Heliotron E
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ABSTRACT: The first pellet injection experiments on NBI-heated current-free plasmas in Heliotron E have been carried out. The pellet and plasma species are hydrogen. The increment of line-averaged density is up to 8.2 × 1019m−3, which is consistent with particle numbers of 1020 contained in a pellet used in the experiment. The behaviour of the plasma is studied in two cases of magnetic field strength, B = 1.9 T and B = 0.94 T. The density after pellet injection decays slowly (50–150 ms) in most cases, but in some cases with B = 0.94 T it decays rapidly (< 10 ms). This may be related to power balance.Nuclear Fusion 01/2011; 25(1):94. · 4.09 Impact Factor -
Article: ICRF heating of currentless plasma in Heliotron E
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ABSTRACT: In the Heliotron E device, a non-axisymmetric helical system, ICRF heating experiments were carried out for the first time, using fast-mode and slow-mode waves. In the fast-wave heating experiment, ICRF power of up to 550 kW was emitted during 15 ms by four antenna loops. Effective heating of a current-less ECRH-produced target plasma was observed over a wide density range. The plasma loading resistance of an antenna loop reached about 5 Ω. This is a value comparable with that of tokamak experiments. The increments of ion and electron temperatures by fast-wave heating were about 200–230 eV at an electron density of about 3 × 1019m−3. Minority heating and pure second-harmonic heating have almost the same efficiency ((1–2) × 1019eVm−3kW−1) during the short RF pulse used (t ≤ 15 ms). The energy transfer rate from the waves to ions and electrons could be explained by mode conversion. The signals of toroidal eigen-modes were experimentally observed and radial mode numbers could be determined using a simple model. In the slow-wave heating experiment, the upper density limit of effective heating appeared to be in qualitative agreement with wave theory.Nuclear Fusion 01/2011; 24(8):1003. · 4.09 Impact Factor -
Article: Behaviour of electrons of an ECRH plasma in Heliotron E
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ABSTRACT: In Heliotron E, a plasma is produced and heated only by an electron cyclotron resonance wave (f = 28 GHz). The output power and pulse width are up to 90 kW and 40 ms, respectively. The parameters of the plasma are in a wide range (Te = 0.3−1.1 keV and e = (1.2−9.5) × 1018m×3). The dependence of the electron temperature on the microwave output power indicates no saturation of the electron temperature. A preliminary analysis is also presented which indicates that the absorption efficiency may be rather good. The anomaly factor, which is the ratio of the experimental particle diffusion to the neoclassical value in the plateau regime, is 2 to 6 for the case of medium density (about 5 × 1018m−3).Nuclear Fusion 01/2011; 23(10):1333. · 4.09 Impact Factor -
Article: Optimum confinement of ECH plasmas in Heliotron E
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ABSTRACT: The optimum confinement of plasmas produced by electron cyclotron heating (ECH) in Heliotron E and some other significant transport characteristics of ECH plasmas have been studied experimentally by changing the magnetic field configuration. It is found that the plasma confinement can be improved by minor changes of the magnetic field configuration. The main results are as follows: (1) Plasmas with a minor radius reduced (by means of a limiter) to two thirds of the value in the original (non-limited) configuration have optimum confinement properties when the magnetic axis is shifted inward by 20 mm. (2) The electron pressure in an optimized plasma configuration (confirmed by neutral beam injection) is higher than that in a standard ECH plasma. (3) The insertion of a carbon limiter causes the plasma line density to increase, with no significant degradation of the core temperature or density. (4) Magnetic field perturbations hardly affect the temperature and density profiles, unless the magnetic axis is shifted outward by a large (50 mm) distance. (5) Transport analysis indicates that significant improvement in the confinement properties can be achieved by slightly manipulating the magnetic field configuration.Nuclear Fusion 01/2011; 31(12):2349. · 4.09 Impact Factor
Top co-authors
Top Journals
Institutions
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1998–2011
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Kyushu University
- Research Institute for Applied Mechanics
Fukuoka-shi, Fukuoka-ken, Japan
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1988–2011
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Kyoto University
- • Graduate School of Energy Science
- • Institute of Advanced Energy
Kyoto, Kyoto-fu, Japan
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2006
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The University of Tokyo
- Graduate School of Frontier Sciences
Tokyo, Tokyo-to, Japan
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